1. Field of the Invention
The present invention relates to an antenna-duplexer and a communication apparatus, used for instance at the microwave band.
2. Description of the Related Art
There is a frequency-variable antenna-duplexer in which an impedance element (switching element), such as a PIN diode or a variable-capacity diode, is connected to a resonator via a capacitor or the like, and the resonant frequency is varied by voltage-controlling these elements. (See for instance Japanese Unexamined Patent Publication No. 7-321509) When a PIN diode is used, the frequency can be switched ON and OFF so that there are two bands, one band during ON and another during OFF. Usually, a positive control voltage is applied to switch the PIN diode ON, and a negative voltage is applied to switch it OFF. A negative voltage is needed for switching OFF because when a high-frequency signal of large electrical power is input, a high-frequency voltage is applied to the PIN diode, thereby switching it ON; a negative voltage avoids this. That is, a negative voltage is needed to prevent the pin diode from becoming unstable when a large power is input, resulting in fluctuating frequency characteristics.
In the above described frequency-variable antenna-duplexer, the impedance element, such as a PIN diode, is nonlinear, and consequently when an unwanted electromagnetic wave enters from the antenna terminal during transmission, the transmission wave F1 and the intrusive wave F2 suffer relative modulation. As a result, a spurious signal F3 (2F1-F2) is generated. Depending on the frequency of the intrusive wave F2, the spurious signal F3 may be the same as the reception frequency. Therefore, in an antenna-duplexer, such relative modulation distortion needs to be reduced as much as possible. This relative modulation distortion occurs when the impedance element has been switched OFF by applying a negative voltage to it.
One method of countering this problem is to increase the negative voltage to stabilize the diode. However, this requires a circuit to generate the large negative voltage, and consequently the communication apparatus cannot be made small-scale. Furthermore, in a third embodiment disclosed in Japanese Unexamined Patent Publication No. 7-321509, two diodes are used for the resonator circuit, instead of generating a large negative voltage. However, since two diodes must be used for all every resonator circuit, there is a problem that an antenna with multiple resonator circuits will be extremely large, making this method inefficient.